The present invention relates generally to position estimation using a satellite system, and in particular, to a method and system for determining the location of a terrestrial-based terminal using pilot signals emitted by a geosynchronous earth orbit satellite.
A conventional position determination can be accomplished using the Global Positioning System (GPS). In the GPS, a plurality of earth orbiting satellites transmit coded radio signals. Earth-based receivers of these signals are capable of determining their positions in an earth-centered coordinate reference system utilized by the GPS. Relying on four GPS satellites, a terrestrial-based receiver can estimate its location to within a few meters rapidly if its initial GPS position and timing delay estimates are known. However, if such initial estimates are not available, it may take up to one minute for the GPS receiver to acquire its position from the GPS signals. Accordingly, there is a need to improve the speed with which initial user location is acquired.
In addition, accurately determining the position of a mobile user permits an accurate estimation of the round-trip transmission delay between the user terminal and the satellite. Time delay estimations are important in time division multiple access (TDMA) satellite communication systems, such as the Geosynchronous Earth Orbit Mobile Satellite System, i.e., the Geo Mobile (GEM) system. In radio frequency (RF) TDMA communication systems, user terminals transmit message bursts at predetermined times corresponding to time slots. To ensure that its burst reaches the satellite at the correct time, a user terminal must determine its xe2x80x9ctimingxe2x80x9d, i.e., when it should transmit, which is a function of the transmission delay. Also, as part of determining its timing, a user terminal must request a time slot assignment from the system prior to establishing a communications link. User terminals can transmit time slot requests during a random access channel (RACH) window. The RACH window is a predetermined interval defined by the system to occur following each regular transmission of a RACH indicator message by the satellite over a broadcast control channel (BCCH). In response to the RACH indicator, a user terminal may transmit its request during the RACH window, which opens at a predetermined time after the RACH indicator.
The duration of the RACH window depends upon the accuracy of the user terminal delay estimations. Accurate delay estimations allow a system to operate with a smaller RACH window. In turn, this permits the system to dedicate more time slots to message bursts, thus increasing the effective capacity of the communication system.
In a communication system such as GEM, a user terminal is in communication with only one geosynchronous satellite at a time. Consequently, only one satellite is available to a GEM user to perform location acquisition. Thus, there is a need for a method and system capable of determining a user position based on transmissions from a single geostationary satellite.
It is an advantage of the present invention to provide a method and system for determining position using the relative signal strengths of a plurality of pilot signals defining spot beams emitted by a single geosynchronous satellite. Another advantage of the present invention is that it allows a user terminal to determine its position without any prior knowledge of its position or delay timing. A further advantage of the present invention is that it can acquire an accurate position estimate in less than three seconds, which can then be used to determine timing delay.
The novel method and system embodying the invention and disclosed herein can include a user terminal (UT) capable of estimating its position using pilot signal relative power measurements of at least three spot beams emitted by a satellite in geosynchronous orbit. Each spot beam corresponds to a respective pilot signal. Also, the pilot signals can be emitted using a phased array antenna included on the satellite.
The UT performs its location determination by comparing estimated relative strengths of the pilot signals to one or more actual relative signal strengths measured at the UT. To estimate relative signal strengths, the UT first obtains the location of the spot beam centers and the satellite from system information carried by the pilot signals. With this information and the attenuation pattern of the satellite antenna, the UT can estimate the relative strength of the pilot signals at corresponding points within the region where the UT resides. Between two or more spot beams, the estimated relative signal strength can be iteratively computed until it converges to the measured relative pilot signal strength. After convergence, the location within the region corresponding to the estimated relative signal strength can represent the approximate location of the UT.
Because a UT in accordance with an embodiment of the invention relies on the relative strength of pilot signals from a single satellite, it does not require prior knowledge of the UT timing or position to determine the UT location. Moreover, since no prior knowledge of location or timing is necessary, the UT can quickly determine its location from a cold start, relative to the acquisition time required by a GPS receiver.
In addition, in contrast to terrestrial-based RF communication systems, such as cellular subscriber systems, a communication system embodying the invention relies on pilot signals emitted from a single source, i.e., a phased array antenna of a satellite. Accordingly, the relative signal strengths are a primarily a function of the shape of the beam emitted by the antenna, rather than the physical distance between RF repeaters.
Furthermore, because the measured pilot signals are emitted from a single source, they take the same path from the satellite to the UT. Consequently, environmental conditions, such as weather, uniformly effect the strength of the pilot signals received at the UT. Since each pilot signal is similarly effected, the relative signal power measured by the UT remains largely unaffected with changes in environment. This permits the UT to accurately determine its location with significantly reduced susceptibility to variations in its usage environment.
The position estimation by the UT can be accomplished using the Geo Mobile (GEM) system. The location determination is useful for: 1) facilitating the Global Positioning System (GPS) position fix; and 2) estimating the differential round-trip delay among the actual user terminal position, the satellite, and the round-trip delay reported in the pilot signal system information to the center of the spot beam. Determining the approximate UT position reduces GPS position acquisition time; while determining the UT delay time allows the GEM system to operate more efficiently.